Synthesis of new semiconductors responsive under visible light/sunlight has been considered as an elegant strategy of photocatalyst development for efficient solar light harvesting applications. In the present study, new La-substituted layered Sillen-Aurivillius oxyhalide intergrowths, Bi3LaNbO8X (X=Cl, Br), have been prepared by solid state reaction. The compounds are characterized by powder X-ray diffraction (p-XRD), field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), UV-vis diffuse reflectance spectroscopy (UV-vis DRS), electrochemical impedance spectroscopy (EIS) and photoluminescence (PL) studies. The P-XRD data revealed formation of Sillen-Aurivillius hybrid perovskites isostructural to the parent compounds, Bi4NbO8X (X = Cl, Br), and the FE-SEM-EDS confirmed the morphology to be agglomerates of submicron-sized particles with desired compositions as expected. The band gap for all the pristine oxyhalides lie in range 2.43-2.54 eV indicative of their sunlight active nature. The sunlight-driven photocatalytic activity studies through Rhodamine B (RhB) degradation unveiled excellent dye degradation efficiencies over Bi3LaNbO8Cl and Bi3LaNbO8Br in the acidic medium. The enhanced photocatalytic activities are attributed to superior charge carrier separation and dye adsorption in the La-substituted phases. Scavenger tests demonstrated the active role of hole and hydroxyl radical as key species in the RhB degradation under sunlight in presence of Bi3LaNbO8Br. The demonstration of enhanced activity under natural sunlight is significant in future development of environmental remediation technologies by harvesting solar energy.